Controls and Switching Circuits

Question 1

What is the purpose of a mechanical interlock on a reversing motor magnetic?

Answer 1

To prevent a short-circuit condition. The mechanical interlocking device makes it physically impossible for both sets of contacts to be closed at the same time

Question 2

What are the terminals for the holding contacts labelled as on a NEMA style start?

Answer 2

Holding Contact Terminals = 2 & 3

Question 3

How are the holding contacts of a motor magnetic connected in relation to the start button?

Answer 3

The holding contact is connected in parallel with the start button

Question 4

What is the purpose of the electrical interlock on a reversing magnetic starter?

Answer 4

The electrical interlock prevents the operating coils from burning out

Question 5

What are the basic componets of a reversing motor magnetic?

Answer 5

1. Two contactors
2. One set of overload devices

Question 6

If a motor starter is used for Jogging operation, how would you adjust the horsepower rating of the starter?

Answer 6

The horsepower rating of the starter must be reduced if it is to be used for jogging purposes

Question 7

How do you size a NEMA motor starer?

Answer 7

NEMA starters are sized according to the conditions of use for the motor

Question 8

Given a certain NEMA sized starter, as the horsepower rating increases, does the voltage rating of the starter increase or decrease?

Answer 8

Increases

Question 9

A circuit is designed with low-voltage release. The overloads in the circuit trip. When the overloads are reset the motor does not restart. Is this a normal condition?

Answer 9

No, it is not

Question 10

If you wanted to add another stop button into a circuit, how would you connect it in relation to the existing stop button?

Answer 10

Stop buttons are connected in series with each other

Question 11

What kind of control circuit would be ideal for a wood-planing machine?

Answer 11

A control circuit that has low-voltage protection properties

Question 12

A float switch is an automatically operated pilot device that is used to detect a change in the level of liquid in a tank. True or False?

Answer 12

True

Question 13

What are the seven sections of a reversing magnetic motor starter?

Answer 13

1. General Stop
2. Forward Start
3. Forward Stop
4. Forward Holding
5. Reverse Start
6. Reverse Stop
7. Reverse Holding

Question 14

What is Jogging?

Answer 14

• Jogging is when a motor is to be repeatedly energized in short burts at rated voltage

Question 15

What is positive jogging?

Answer 15

• Positive jogging is a connection method that prevents the motor from accidentally going into run if the button is released very quickly

Question 16

What is Inching?

Answer 16

• Inching is when a reduced voltage is used to allow a machine to creep to a desired position

Question 17

What are three ways to achieve jogging?

Answer 17

1. Selector switch jogging
2. Pushbutton jogging
3. Control relay jogging

Question 18

Which method of jogging could allow the magnetic coil to hold in if the pushbutton is operated very quickly?

Answer 18

Pushbutton jogging

Question 19

Why must the selected starter rating be greater when a motor is to be jogged?

Answer 19

• The starting current of a three-phase induction motor can be 2-8 times its full load current
• During starting a lot of stress is placed upon the contacts of the starter and other equipment, so it is necessary to upsize the starter

Question 20

Which IEC Use Category does jogging motors fall under?

Answer 20

• AC-4

Question 21

What might a pilot light indicate?

Answer 21

1. Power on to a motor controller
2. Contactor energized
3. Overload trip of the overload relay